The occurrence of high-fluorine, high-alumina silicates in nature has been well documented in the scientific literature. Topaz, tourmaline, and amphiboles such as glaucophane represent examples of this class of minerals. Those silicates exhibit such properties as high hardness, pleochroism, piezoelectricity, piroelectricity, chemical durability, and thermal conductivity which strongly recommend research to investigate their utility in advanced ceramic and electro-optic applications.
Topaz, one of the principal fluorine-bearing minerals, comprises a solid solution between a fluorine end member, Al.sub.2 SiO.sub.4 F.sub.2 (fluor-topaz) and a hypothetical hydroxyl end member, Al.sub.2 SiO.sub.4 (OH).sub.2. Naturally-occurring topaz contains significantly greater amounts of fluorine than hydroxyl, with samples ranging from nearly pure fluor-topaz, containing approximately 20% by weight fluorine, to a stoichiometry of about Al.sub.2 SiO.sub.4 F.sub.1.4 (OH).sub.0.6, containing approximately 13% by weight fluorine. Topaz is an orthosilicate of simple composition which typically crystallizes in orthorhombic symmetry, with cell parameters of a .apprxeq.4.65 .ANG., b=8.79-8.83 .ANG. (increasing as OH is substituted for F), and c.apprxeq.8.39 .ANG..
The physical and chemical properties of topaz render it a prime candidate for the formation of protective films. It exhibits a Mohs hardness of 8, below alumina and diamond only; it demonstrates extremely high chemical durability, even in concentrated acids including HF; its thermal conductivity of 55.9, which is quite comparable to that of alumina, is unusually high for a silicate; its density of 3.5 g/cc, although high for a silicate, is less than that of alumina and steels; and its exceptionally high level of fluorine suggests its utility in applications, such as culinary ware, where non-sticking characteristics are highly desirable.
The production of highly fluorinated ceramic materials has not been commercially practical because the extreme volatility of fluorine makes it difficult to introduce high concentrations of fluorine into ceramic powders or glasses, which difficulty is compounded when those powders are then sintered into an integral body or the glass is heat treated to crystallize it in situ.
Accordingly, the principal objective of the present invention was to devise a method for synthesizing crystals having a stoichiometry approximating that of topaz.
A specific objective of the instant invention was to devise a method for synthesizing crystals having a stoichiometry approximating that of topaz with a fluorine content between about 3-21%.